Electric lamp, light diffusing coating therefor and method of manufacture



March 20, 1951 M. PlPKlN 2,545,895

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OFMANUFACTURE. Flled Jan '7, 1948 4 Sheets-Sheet 1 i MM H InvenTor: MarvinPipkin,

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PERCENT OFJNTENS/TV March 20, 1951 M PlPKlN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OFMANUFACTURE Filed Jan. 7, 1948 4 Sheets-Sheet 2 50. CM. Q, Q Q

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March 20, 1951 M. PIPKIN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OFMANUFACTURE Flled Jan 7, 1948 4 Sheets-Sheet 3 lnven bor: Marvin Pipkin,

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March 20, 1951 M. PIPKIN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OFMANUFACTURE Filed Jan. '7, 1948 4 Sheets-Sheet 4 lnvn tovz MarvinPipkin,

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Patented Mar. 20, 1951 ELECTRIC LAMP, LIGHT DIFFUSING COAT- ING THEREFORAND METHOD OF MANU- FACTURE Marvin Pipkin, Cleveland Heights, Ohio,assignor to General Electric Company, a corporation of New YorkApplication January 7, 1948, Serial No. 878

32 Claims. 1

My invention relates to incandescent electric lamps and diffusingelectric lamp bulbs or enclosures, and methods of producing the same.

This application is a continuation-in-part of my copending patentapplication Serial No. 728,769, filed February 15, 1947, now abandonedand which is assigned to the assignee of this application.

Light-diffusing bulbs have been produced heretofore by using opal glass,that is, glass which is rendered opalescent by the addition of fluoridesor phosphates thereto during the manufacture thereof. However, such aglass is expensive, and the light absorption thereof very high,amounting to something like 25 or 30% in bulbs having sufficiently highdiffusion to effectively mask the filament or other light source.Moreover, the light absorption is not uniform due to the uneventhickness of the bulb, and this results in the light emitted from thelamp being of quite low and unequal intensity. Such bulbs have thereforehad very limited use.

Light-diffusion has also been obtained by enamel coatings on theexterior surfaces of the bulbs. However, such coatings are ratherexpensive and they also absorb a considerable amount of the lightemitted from the lamp, amounting to more than even for white coatings.Moreover, the exterior coatin tends to collect dirt easily. On the otherhand, the

application of such coatings to the interior surface of the bulbs hasalso failed to be accepted generally because of high absorption as wellas the tendency to thereby introduce impurities which are injurious tothe lamp. Such bulbs have therefore had only a limited'use.

It has also been proposed heretofore to coat the inside surface ofelectric lamp bulbs by burning metals, and silicon, in powdered formoutside the bulbs and then forcing the particles into the bulb where theparticles were dusted on the inside surface thereof. However, theseoxides did not produce efiicient or satisfactory coatings, and have notbeen put to any practical use in lamp manufacture due to the fact thatthe kind or state of the material used, and the method of application orburning, were not such as to make possible control of the particle sizeof the coating, and would not permit uniformity or adherence of thecoating to the extent where such coatings could be employed in apractical electric lamp. Moreover, when it was attempted to burnsilicon, it was found to burn very violently, producing sputtering andeven explosive flashing, which characteristics make it extremelydiflicult, if not practically unusable, because of the incompletecombustion of the silicon. The collected particles were not of properform, size, configuration, and the incompletely burned flocculentparticles of crystaline silicon were not sufficiently adherent and wereblack, grey or brown in color, which factors prevented use of such acoating as a light diffusing medium in an electric lamp.

The most widely used diffusing bulb has been that in which the insidesurface was frosted by etching. That process is quite inexpensive, andthe bulb has relatively high diffusion with very low absorption of thelight fromthe lamp. Notwithstanding the wide use and generalacceptability of this type of diffusing medium there has been evidenceda need for improved and even more efficient light-diffusing moans.

One of the objects of the present invention is to produce lamp bulbs andother devices or enclosures having very high difiusion with minimumlight absorption. Actually, I am able to obtain lamp bulbs with veryhigh diffusion and, apparently very small or no measurable absorption,which is indeed surprising and contrary to what has been experiencedheretofore with diffusing lamp bulbs.

Another object is to provide a lamp bulb having on its interior surfacea superficial lightdiifusing coating. Another object is to provide asimple and inexpensive method of applying such a coating.

Another object is to provide an internal lightdiffusing coating which isnot injurious to the lamp but, as a matter of fact, is actuallybeneficial.

A further object is to provide a previously etched bulb or other surfacewith a coating which will make the bulb very highly diffusing withvirtually no absorption of light due to the added coating.

A still further object is to provide a highly efficient and highlydiffusing coating for illuminating ware generally, such as lightingfixtures.

It is. a still further object of my invention to provide a new andimproved incandescent electric lamp which not only provides a highlyefficient light diffusing coatin which substantially completely masksthe incandescent filament but which also makes it possible to operatethe filament at a higher temperature, by virtue of the use of a gaseousatmosphere consisting essentially or exclusively of argon, krypton orxenon, or mixtures thereof, thereby obtaining a higher lumen output andreducing or completely compensating for the slight loss due to thediffusing coating.

Further objects and advantages of my invention will appear from thefollowing description of species thereof and from the drawing.

In the drawing, Fig. 1 is an elevation of one species of apparatus whichmay be used to coat lamp bulbs in accordance with my invention; Fig. 2is an elevation in section, of a modified burner; Fig. 3 is an elevationof a further modification of coating apparatus; Fig. 4 is an elevation,in section, of the burner portion of the apparatus shown in Fig. 3; Fig.5 is an elevation, partly in section, of an incandescent lamp having abulb coated in accordance with my invention; Figs. 6 and 7 are graphsshowing certain characteristics of lamps comprising this invention; Fig.8 is an elevation of another form of lamp embodying my invention; Fig. 9is a somewhat diagrammatic elevation, partly in section, of a lampfixture embodying my invention; and Fig. 10 is a "size-frequency ofoccurrence" curve which is approximately representative of some coatingsmade in accordance with my invention. Figs. 11 and 12 are reproductionsof actual electron micrographs of silica coatings made in accordancewith my invention and the specimens for which were prepared from insidefrosted surfaces of electric lamp bulbs. Fig. 13 is a reproduction of anactual electron photo-micrograph showing the particle arrangement of asilica coating produced in accordance with my invention and which wascollected directly on a resin film in order to facilitate observationunder an electron microscope and without necessitating removal from aglass surface forobservation under an electron microscope.

Generally speaking in accordance with my invention, I provide a highlyefiicient radiation or light diffusing coating. For example, thiscoating may be placed on alight transmitting body or light diifusingbody, and comprises finely divided particles of silica formed by thecombustion of an inflammable silicon compound. In the combustion of suchcompounds, there may result gaseous products of organic materials, or asmoke or fume, such as a suspension in a gas of particles of silicawhich are ultimately collected by or deposited .on the body to be coatedand are adherent thereto. The above objectives are attained by thecombustion of an inflammable silicon compound which will leave a depositof extremely fine particles of substantially pure silica adherent orfused to the inner surface of the bulb, or other object to be coated.

The coating density, or the weight of the coating in milligrams persquare inch, will of course vary according to the nature and conditionof the surface or material to be coated, and according to thelight-diffusing characteristics desired. Furthermore, the density of thecoating to obtain specified light-diifusing characteristics may berelated to the size of the silica particles to obtain benefit-of bothmaximum diffusion and minimum absorption. I have found that there is anoptimum range of coating densities (which of course establishes coatingthicknesses) and that there is an optimum range of particle size inorder to obtain maximum diffusion with minimum or negligible absorption.

In addition, I have found in the coating of glass or vitreous articlessuch as clear glass and inside-frosted electric lamp bulbs that veryhigh or maximum light diffusion with minimum absorption is obtained byusing coating densities (that is, milligrams of silica per square inch)preferably ranging from a fraction of a milligram, such as milligram, toseveral milligrams per square inch, although coating densities rangingto about 20 milligrams per square inch may be beneficially employeddepending upon the nature of the material to be coated and thelightdiifusing characteristics desired.

Moreover, in connection with particle size of the silica, it has beenfound that optimum lightdifiusion with minimum or negligible absorptionmay be obtained by using silica particles the dimension or diameter ofwhich is a fraction of a micron and preferably wherein the averagediameter of the particles, as hereinafter defined, is in the range ofabout to micron, the individual particles ranging in size from about 0.9micron to 30 angstrom units and less. It will be understood that onemicron is equal to 10,000 angstrom units. For example, I have madeelectric lamps provided with silica coatings in which satisfactorylight-diffusion and minimum absorption are obtained and wherein theindividual particle sizes range from less than 30 angstrom units toabout 7150 angstrom units.

The average diameter referred to herein is the average particle sizewith respect to surface which, for a given sample, is the diameter ofthe uniformly sized particles which would give the same total surfaceper unit volume of sample. This average diameter is calculated as wheren is the number of particles in any given size class and d is thediameter representing that class. Some lamps made in accordance with myinvention show a size-frequency of occurrence curve of the typeapproximating that shown in Fig. 10 and which is derived by plotting thenumber of particles in any given size class against the diametcrrepresenting that class. For those desir-' ing further detailedinformation relative to the above method of analyzing and studying thefrequency of occurrence-particle size characteristics of microscopicmaterials, reference may be had to the Handbook of Chemical Microscopy.by Chamot and Mason, vol. I, second edition, published 1938 by JohnWiley & Sons, Inc., particularly pages 416-419 thereof. The numericalaverage diameter or arithmetical mean diameter of the particles is, ofcourse, less than the above defined average diameter (113) with respectto surface. In coatings made in accordance with my invention frominvestigations and studies the coatings are indicated as comprisingparticles the numerical preponderance of which have a dimension or sizeless than the shortest wavelength of light or visible radiation, that isless than about 4000 angstrom units.

Electron-microscope investigations have been made to establish anddetermine the particle sizes of coatings which afford maximumlightdifi'usion with minimum or negligible absorption. These and furtherstudies have furnished additional information as to the configurationand state of the silica particles. Apparently the silica particles arerounded-or in the form of substantially perfect spheres, and the silicais in the non-crystalline or amorphous state.

The limit of resolution of the electron microscope employed in makingthese studies was 30 angstrom units, indicating that there are manyparticles of silica having a size less than this dimension. Moreover, astudy of the electron microscope pictures taken of the silica coatingindicate that there is a very thin layer of very small particlesdirectly adherent and fused to the glass bulb, and that this under-layercannot be easily rubbed off or removed, although the superimposed orupper layers can be removed by rubbing or abrasive action. In thismanner it will be appreciated that so far as the silica coating isconcerned it may be considered to be partially fused or fritted to asupporting body such as an incandescent lamp bulb being coated. The

underlayer, in itself, affords appreciable diffusion properties.

I have found that I can obtain degrees of diffusion such that aclear-glass bulb lamp coated with silica in accordance with myinvention'may have a maximum brightness of the order of a fraction of aper cent to a few per cent of that of an otherwise similar lamp havingno coating, while an inside-frosted bulb lamp coated in accordance withmy invention may have a maximum brightness of the order of a smallfraction of a per cent of that of an otherwise similar clear-bulb lamp.

I have also found that, in coating light transmitting bodies andlight-diffusing bodies, I may control the general light diffusingcharacteristics of a silica coating by controlling the particle size.Among the factors which may be considered effective in establishing theparticle size to obtain desired light-diffusing characteristics of thecoating, I have found that the following are of effect: concentration ofthe silicon compound, temperature of the flame or the region immediatelysurrounding the flame, the smoke or fume density, the rate of cooling ofthe gaseous combustion products or the smoke, and the concentration ofoxygen or the oxygenous concentration if a combustion supporting gas isemployed.

It has been appreciated for many years that it is desirable to operate afilament of an incandescent lamp in a gas to retard evaporation of thefilament and to increase the filament and lamp life. However, the gas soused, by virtue of its thermal conductivity, and energy loss due to thethermal convection losses, results in a reduced filament operatingtemperature.

Of course, it has also been appreciated heretofore that the inert gasesare highly desirable as fillings for incandescent lamps. Moreover,nitrogen has been employed as a filling gas. Furthermore, it has beenthe practice for many years to use as a filling gas various mixtures ofinert gases, such as argon, with nitrogen. However, it has been usualpractice in the manufacture of electric lamps to use in all suchmixtures a relatively large percentage of nitrogen in order that themixture have a sufficiently high breakdown voltage characteristic toprevent the establishment of an electric discharge or breakdown acrossthe filament terminals inside the bulb, or across any portion of thefilament. While nitrogen does have a breakdown voltage greater than theinert gases, it has a greater thermal conductivity thereby resulting insubstantial conduction and convection losses with the consequentreduction in luminous efilciency.

The silica coating which I provide is not only a very efficient lightdiffusing medium but is also a means instrumental for recovering any.loss in lamp efliciency occasioned by the light diffusing function ofthe coating, by making it possible to operate the incandescent filamentat a higher temperature without increasing the probability of voltagebreakdown across the filament terminals. A very high percentage of thesilica particles of the above described silica coating are directly andself-adherent to the interior surface of the lamp bulb. By virtue of itsadherence and its finely divided nature, the silica coating serves as acontamination prevention means by covering the glass bulb interiorsurface thereby holding any impurities, such as free caustic soda orcarbonates which are always present in lime glasses used in lampmanufacture, and which may otherwise be present on the inside of theglass bulb and which upon dislodgment tend to cause a voltage breakdownacross the filament terminals by constituting an undesirable impurity inthe gas filling. An inert gas having an atomic weight greater than 39,such as that of the group consisting of argon, krypton and xenon, ormixtures thereof, may be used substantially exclusively as, or as a veryhigh percentage of, the gas filling thereby taking advantage of thelower thermal conductivities of these gases as well as their greaterretarding effect on thermal evaporation, and thereby obtaining higheroperating filament temperature and lamp efficiency,

without causing arc discharges within the lamp bulb due to voltagebreakdown even under rough usage or lamp handling. If an attempt is madeto lower the nitrogen content of present day lamps, it is found thatsuch lamps under rough usage tend to form are discharges due todislodgment of impurities from the bulb wall. Where it is desired toraise the breakdown voltage of a gas filling to a voltage greater thanthat of the inert gas or mixtures alone, a small percentage of nitrogen,not exceeding 5 per cent by volume, may be used. For example, I may usea gas filling consisting of about 98 per cent argon and 2 per centnitrogen, by volume, in an incandescent lamp having a glass envelopeprovided with a silica coating on the interior surface thereof, therebyproviding improved light diffusion without any loss .in lamp efficiency.

In coating a bulb or other illuminating ware, I oxidize any of certainvolatile or gaseous silicon compounds by burning, thereby breaking downthe molecule and forming a fume of silica. These fumes are collected onthe inner surface of the bulb, for example, where they form aselfadherent coating of very pure finely divided silica which containsnothing of a harmful nature, not even water of hydration, that mightadversely affect the performance of the lamp. Although the coating maybe rubbed off with the fingers it is more than sufficiently adherent toWithstand the efiect of jarring or other rough handling, therebydispensing with the need for a binder of any kind which might introduceharmful impurities into the lamp. However, the adherence of the coatingmay be increased, if desired, by steaming it, i. e., exposing it to aflow of steam for a brief time. 4

I prefer to use silicon compounds Whose molecules contain no atoms otherthan silicon, carbon, hydrogen and oxygen. The presently preferredmaterial is ethyl orthosilicate (C2H5)4Si04, which is an oily liquid.Other compounds include silicomethane, silicoethane, silicopropane,silicobutane, silicopentane, disiloxane, methyl silicate, methylsilicane, ethyl silicane, dimethyl silicane, diethyl silicane,tetramethyl silicane, ethoxytriethyl silicane and the like. These aregiven merely as examples and not by way of limitation, since many othermaterials might be used. However, in general it is advisable to avoidthe use of materials which contain too much 7 organic matter since thesemight cause a deposit of carbon to be formed.

Certain gaseous compounds such as silicon hydride or silicanes orsilanes containing only silicon and hydrogen ignite when exposed to airor oxygen. Thus, in coating a bulb, a suitable burner can be placedinside the bulb and when such compound is released with oxygen flowingit will be ignited. A definite density of coating can be obtained byregulating the amount of such compound admitted.

One manner of carrying out the process on a lamp bulb is by impregnatingany suitable form of wick with a volatilizable liquid compound such asethyl silicate, and burning the compound inside the bulb in the presenceof a stream of air or oxygen to insure continuing combustion and tocarry the products of combustion other than silica out of thefbulb.

As applied to lamp bulbs, the coating may be sults, and even betterILSllltS are obtained when the coating is applied to a bulb whichpreviously has had its inside surface frosted, preferably by etching asdescribed in my Patent No. 1,687,510.

As illustrated in Fig. 1 of the drawing, the burner or wick may consistof a ball or wad I of glass thread wound on the end of a wire rod 2. Thewad is dipped into the ethylsilicate, and the rod is then inserted in atube 3 through which a stream of oxygenous gas is blowing. I prefer touse pure oxygen, rather than air, for example, in order to insurecomplete combustion and subdivision of the particles of smoke andthereby avoid deposits of flocculent silica. The wad is then lighted andthe bulb 4 is placed over the burning wad and rotated on a hollowtubular chuck 5 which surrounds the tube 3 and terminates at its upperend in a thin, resiliently compressible collet portion which grips thebulb. A dense white smoke is emitted consisting of silicon dioxide (1.e., silica), water vapor and carbon dioxide, the silicon dioxide beingdeposited on the inner surface of the bulb in a smooth adherent film,and the water vapor and carbon dioxide being blown out of the bulb anddownward through the interior of the chuck 5. The density of the coatingmay be controlled either by removing the bulb after a predetermined timeor by initially applying to the wad l a predetermined amount of theethyl silicate.

For a 100-watt incandescent lamp bulb having a volume of some 11 cubicinches and a surface area of about 28 square inches, a coating ofsatisfactory density may be obtained by applying about 11/ or 2 grams ofethyl silicate to the wad I and permitting it to burn until the flamehas burned itself out before removing the bulb. A stream of oxygenflowing at the rate of about 100 cc. per second is satisfactory, with aspeed of rotation of the bulb of about 70 R. P. M., for example.

In the apparatus illustrated in Fig. 1, the hollow chuck 5 is rotatablymounted on a bracket 6 and is driven from any suitable source of powerthrough a belt I encircling a pulley 8 attached to the chuck. The tube 3is adjustably supported by a clamp 9 extending from a suitable base II!which also carries the bracket 6. As illustrated, the tube 3 ispreferably adjusted so that the wad is approximtaely at the center ofthe spherical portion of the bulb.

In Fig. 2, I have illustrated a modified set-up wherein the wad isreplaced by a receptacle or cup II, of metal for example, having anippled bottom. I2 set in a metal sleeve or cup I3 at the applied to aclear glass bulb with very good re- 8 upper end of a rod H which issupported in the tube 3 from any suitable stop or projection at theinside of the tube 3 or by having a portion thereof provided with anoffset bend to frictionally engage the tube. The cup II contains anysuitable absorbent or porous material, which may be a fine powder, forinstance the residue of silica such as that deposited on the exterior ofthe tube 3 in carrying out this process. The ethyl silicate i pouredinto the cup and ignited as described above. With this arrangement it ispossible to use less ethyl silicate, for example about /2 gram ascompared with 2 grams in the case of the wad I.

In Figs. 3 and 4.1 have illustrated a further modification of theapparatus shown in Fig. 1. In this case the burner comprises a pluralityof nested sheet metal cups I5 similar to that shown in Fig. 2 exceptthat they are open at the bottom. The nippled bottom portions I6 therebyconstitute a series of baffles. The lower-most cup I5 is fitted tightlyinto the upper end of a comparatively fine metal tube I! which extendsthrough the interior of the oxygen supply tube 3. The tube or conduit IIis connected at the bottom thereof to a tube I8 which communicates withan outlet I9 in the bottom of a constantlevel reservoir 20. The level ofthe inflammable liquid silicon compound 2| in the reservoir 20 ispreferably above the top of the burner I5, and the fiow of liquid 2| tothe burner I5 is regulated by any suitable means such as a valve 22 inthe outlet I9 to provide a constant flow of the'liquid.

The oxygen is supplied to the tube 3 through a T fitting 23 whichsurrounds the tube I1 and which is connected to the lower end of tube 3by a rubber nipple 24. The lower end of the T is closed by a rubbernipple 25 which surrounds the lower end of the T and a portion of thetube I1. The oxygen therefore flows around the tube I1 and within theinlet tube 3.

In the device shown in Fig. 3, the burner I5 operates continuously. Theflow of oxygen around the burner I5 causes the liquid therein to burnwith a comparatively short flame issuing from the upper cup I5, the sizeof the flame being controlled by the rate of flow of oxygen and ethylsilicate. A deposit of silica tends to build up on the top of the burnerI5 in the form of a cone or cylinder. This deposit may be periodicallyremoved in any convenient manner, as by slicing or cutting it oil". Incoating bulbs of the 100- watt size the ethyl silicate 2| may be fed tothe burner I5 at the rate, for example, of about 72' drops, or 3 00.,per minute, with an oxygen flow of about cc. per second. Each of thebulbs 4 may be exposed to the silica fumes for a period of approximately15 seconds.

The processes described herein are obviously adaptable to massproduction on an automatic machine having a number of heads eachcorresponding to the arrangement shown in Figs. 1

or 3, for example, and mounted on a rotatable indexing turret or othermovable carrier. It is merely necessary for an. operator to periodicallyreplace previously impregnated wads I or cups II as the heads pass theoperator's station.

In Fig. 5, I have shown a -watt incandescent lamp in which the bulb 4,either of clear glass or inside frosted, is provided on its insidesurface with a coating of extremely finely divided silica indicated bythe stippling 26. The filament and other internal parts of the lamp arerepresentative of the ordinary commercial incandescent lamps. Thefilament 21 of coiled-coil tungsten wire type is mounted on and betweena pair of leading-in wires 28, 28 which extend through the conventionalglass stem 29 fused to the neck of the bulb 4. The wires 28 extend torespective contacts on the conventional base 30. The bulb I may befilled with inert gas such as argon.

A remarkable feature of bulbs coated in accordance with my invention isthat the thickness or density of the coating can be increased greatlyand yet the illumination is extremely uniform, much more so than even inthe case of bulbs of opal glass. Moreover, the coating has no adverseefl'ect on the strength of the bulb or the operation of the lamp, andthe bulbs blacken to a lesser extent during the life of the lamp thanclear or inside frosted bulbs.

The small amount of absorption is shown by tests on 120 volt, 100-wattincandescent lamps. The lamps in the various groups in the table, allwere made under similar circumstances and incorporated the same physicalfeatures except as noted, and all used a gas filling of 88 per centargon and 12 per cent nitrogen.

Table [Averages based on large numbers of lamps] 10 lumen per watt, or4.9% for the silica coated inside frosted bulbs.

The area of the effective bulb surface (that is, above the plane S-S,Fig. 5, which represents the line along which the stem 28 is sealed tothe bulb) is approximately 28 square inches, so that the average densityof the coating of to 80 mg. total weight is about 1.4 to 2.86 milligramsper square inch. But good results may also be obtained with coatings ofabout 20 mg. total weight, i. e., 0.7 mg. per square inch.

The high degree of diffusion of bulbs coated in accordance with thisinvention is illustrated in Fig. 6 by the curves of brightness of 100-watt tungsten filament lamps of the type shown and described inconnection with Fig. 5, which are based on measurements of horizontalbrightness in candles per square centimeter versus distance incentimeters from the top of the bulb along a vertical line in front ofthe lamps. These measurements were made in a vertical plane through thelamp axis and perpendicular to the plane of the leads 28, 28. The curveA is for a standard inside frosted lamp, curve B for a It is striking tonote that the efficiencies of the lamps with clear bulbsinside-silica-coated, and inside-frosted-inside-silica-coated bulbs,show about the same efiiciency, that is, the same initial lumens perwatt. This result has been shown by tests made on large numbers of lampsin each category, and shows that virtually completely diffusing bulbsfor lamps can now be made which show no, or a very small, loss in lightoutput.

A treatment such as those described above provides the bulb with a veryhighly diffusing coating weighing some 40 to 80 milligrams. Althoughsuch a coating provides eminently satisfactory diffusion, particularlywhen applied to a previously inside frosted (etched) bulb, and show verylittle light absorption, I have prepared lamps with both clear andinside frosted bulbs which were provided with an extra heavy insidesilica coat in order to determine what the absorption would be with muchheavier coatings. To this end, I repeated the coating treatment in eachof those bulbs seven times, resulting in a coating weighing from 280 to560 milligrams, that is a coating density from 10 to 20 milligrams persquare inch. Test results showed that forsix lamps with inside frostedbulbs the total average lumens per lamp were 1699 and the average lumensper watt were 16.60. For the lamps with clear bulbs inside silica coatedseven times, the average for six lamps was 1606 total lumens and 15.73lumens per watt. For six lamps with inside frosted bulbs, inside silicacoated seven times, the average was 1616 total lumens and 15.79 lumensper watt. These results show a loss in average total lumens of 93, or5.5% for the silica coated clear glass bulbs, and 83 lumens, or 4.9%,for the silica coated inside frosted bulbs. The loss in average lumensper watt was .87, or 5.3%, for the silica coated clear glass bulbs, and.81

' lamps.

has been reduced, respectively, about 3-fold and a l6-fold, and thiswith no measurable loss in light output. Since the maximum brightnessfor inside-frosted lamps is below 4.5% of that of clear bulb lamps ofthe same wattage (Patent 1,687,- 510), this means that the maximumbrightness of the silica-coated clear-bulb lamps and the silica-coatedinside-frosted lamps is below 2% and 0.3%, respectively, of that ofclear-bulb lamps of the same wattage.

Measurements on 100-watt lamps of the same type as those referred toabove except that they had clear glass bulbs, showed a maximumbrightness averaging 1289 candles per square centimeter. Thus, themaximum brightness of the silica-coated clear-bulb lamps and thesilicacoated inside-frosted lamps was about 0.9% and 0.15%,respectively, of that of the clear-bulb lamps.

Fig. '7 shows spectral distribution curves of radiation emittedperpendicular to the plane of the leads from 100-watt tungsten filamentlamps with and without silica coatings. Curve D represents a regularinside-frost lamp, curve E an inside frost lamp with silica coating, andcurve F a clear-bulb lamp with silica coating. It will be apparent thatthe distribution in all cases is quite similar.

The effect on particle size of varying the flow of oxygen, and,therefore, the character of the 11 flame, is illustrated by thefollowing results obtained on inside frosted 100-watt bulbs with theapparatus illustrated in Figs. 3 and 4. In one case, a fiow of ethylsilicate at the rate of 120 drops per minute, with an oxygen flow of 468cc. per second, and an exposure of 30 seconds to the smoking flameproduced a coating weighing 41 milligrams in which the particles had anaverage diameter ((13) of 2400 angstrom units, the maximum diameterbeing 4500 angstroms and the minimum diameter being less than 40angstroms. Fig. 11 is a. reproduction of an actual electron micrograph(20,000)!) of the silica coating from the inside surface under theconditions stated and shows the configuration of the particles asmechanically suspended in nitrocellulose for electron microscope study.Fig. 13 shows the type or kind of random distribution of the silicaparticles and the arrangement thereof. In another case, with afiow ofoxygen of 107 cc. per second, a flow of ethyl silicate of 120 drops perminute, and an exposure of 30 seconds, the weight of the coating was 40milligrams, and the particles had an average diameter (dz) of 4300angstroms, the maximum diameter being 7150 angstroms and the minimumdiameter 100 angstroms. Fig. 12 is a reproduction of an actual electronmicrograph (20,000X) of the silica coating deposited on the insidefrosted surface of the latter bulb and removed and mechanicallysuspended in nitrocellulose for analysis under an electron microscope inorder to show the shape of the individual particles. The latter bulb wasconsiderably more diffusing than the former; in

fact, it contained more silica than was necessary for good diffusion.

In another bulb made as described in connection with Fig. l, the averageparticle diameter (dz) was 3400 angstroms, with a range of indi vidualparticle sizes from 6000 angstroms to less than 30 angstroms.

The frequency of occurrence of the particles in any given size class forthese coatings is approximately as shown by the curve in Fig. 10.

I have found that the diffusion characteristics of the silica coatingmay also be controlled after deposition by wetting or exposing thecoating to a liquid or vapor, such as steam. When a liquid or vaporcomes in contact with these particles of silica, the particles areapparently ruptured and they assume a heterogeneous form and arrangementaffording less diffusion. In substance, the particles so treated form athinner layer and the light diffusion is less. Accordingly, the ultimatediffusing characteristics of a silica coating may be controlled byexposing the deposited silica coating to moisture, thereby providing aconvenient method of determining the characteristics desired of theentire coating or a part thereof.

The silica which I employ as a light diffusing element in anincandescent lamp also makes it possible to obtain the improveddiffusion in a lamp without any loss whatsoever in over-all lampefficiency (lumens per watt). Even though, as indicated above, the lossin efliciency due to the silica coating is small, I have found that anyloss so incurred may be completely recovered by using filling gascompositions which heretofore have been considered unsuitable in lampmanufacturing practice.

As stated generally above, it has been the usual practice in mostgeneral purpose incandescent lamps to use a relatively high percentageof nitrogen as a constituent with an inert'gas as the gas filling. Thisuse of nitrogen. in spite of tel its high thermal conductivity, has beendeemed necessary and desirable because of the hi h breakdown voltagecharacteristic of nitrogen. In other words, in order to obtain aresultant or over-all high breakdown voltage of the gas composition, theincident greater heat loss by use of nitrogen was accepted, even thoughit entailed a reduction in lamp efficiency as compared with the use ofan inert gas alone.

A gas filling consisting of per cent argon and 10 percentage nitrogenhas a breakdown voltage which is four times that of pure argon. Statedin other words, the breakdown voltage of pure nitrogen is about tentimes that of pure argon under the same conditions. Fo this reason, itwill be understood that heretofore in order to obtain freedom frominternal arcs during rough handling of lamps, it has been the practiceto use a relatively high percentage of nitrogen as a componentin thefilling gas. For example, in one commercially standardized line of wattlamps, it has been and is the practice to use a filling gas consistingof 88 per cent argon and 12 per cent nitrogen.

Lamps as constructed above. using a relatively high percentage ofnitrogen, that is ranging from 10 to 20 per cent, of course have beensatisfactory from the standpoint that these lamps are free from internalarcing even under rough usage. However, as stated, by virtue of therelatively high percentage of nitrogen and its higher thermalconductivity as compared with the inert gases such as argon, krypton andxenon, it was necessary to accept the incident greater heat loss therebynecessarily imposing on the lamp 9. loss in efiiciency with respect tothat theoretically possible.

By using a gaseous atmosphere consisting essentially of an inert gas,such as argon, krypton or xenon, or mixtures thereof, either with orwithout a small percentage of nitrogen, or by using a gaseous atmosphereconsisting exclusively of an inert gas such as argon, krypton or xenonor mixtures thereof, and by the use of the silica coating I am able toprevent voltage breakdown or are discharges across the filamentterminals, whereas in similar lamps using the same gas composition inthe absence of silica a large percentage of the lamps arc-overinternally. In this manner, I effect operation of the filament at ahigher temperature affording a gain in efficiency over that present inan otherwise similar lamp using a relatively high percentage of nitrogenas a constituent in the filling gas.

When-the silica coating which I provide is employed in a lamp it hasbeen found that the amount of nitrogen employed may be substantiallyreduced to an amount heretofore considered unusable in a practical lamp.Although the percentage of nitrogen employed with the stated inert gasesmay vary depending upon the type of application for which the lamp isdesigned, I have found that the amount of nitrogen by volume may bemaintained at a relatively minor percentage, not exceeding 5 per cent byvolume of the gas composition. For example, in a 100 watt lamp as hereindescribed, I prefer to use a gas composition consisting of an inert gas,such as argon, constituting 98 per cent by volume, and 2 per centnitrogen.

As a standard, reference may be had to a commercial line of 100 watt,volt incandescent lamps employing inside frost and using 88 per centargon and 12 per cent nitrogen. The luminous efliciency of such lamps,made in large quantities for commercial production, in lumens per wattis about 16.3. Lamps built in accordance with my invention employinginside frost and an inside coating of silica as herein explained, andusing a gas filling consisting of 98 per cent argon and 2 per centnitrogen, also have an efficiency of 16.3 lumens per watt, indicatingthat the same amount of light is emitted and the same efficiencyobtained, while obtaining greatly improved diffusion. Stated in otherwords, any loss in efficiency occasioned b the use of the silicacoatirig is completely recovered and compensated for by using the higherpercentage of the stated inert gases.

Furthermore, extensive tests conducted on lamps employing the insidesilica coating and using a gas filling, consisting of 98 per cent argonand 2 per cent nitrogen, show that these lamps are entirely free ofinternal arcs even under the most rough usage. More particularly, in oneset of tests conducted on 57 regular lamps employing inside frost and 98per cent argon and 2 per cent nitrogen, one third of the lamps developedinternal arcs under the rough usage or fragility test. On the otherhand, in similar tests conducted on the same number of lamps having aninside frost and an internal coating of silica and using a gas fillingconsisting of 98 per cent argon and 2 per cent nitrogen, no lamps showedinternal arcing. This test is merely representative of a large number ofsimilar tests so conducted which conclusively show that the silicacoating prevents the internal arcing of the lamps even under roughusage.

The pressure of the gas filling is not critical in carrying out myinvention so that I may use any practically feasible pressure providingthe necessary safety in use. For example, the pressure may be less thanone atmosphere, or greater than one atmosphere. In some general purposelamps the cold gas pressure is about 600 mm., and during operation thegas pressure approaches or reaches about one atmosphere (760 mm.).

In Fig. 8, I have illustrated the invention as applied to the insidesurface of the outer bulb 3| of a high pressure mercury arc lamp whichmay be of the type disclosed in Patent 2,094,694 to C. Bol et a1. andwhich is assigned to the assignee of this application. The bulb 3|encloses a quartz mercury arc tube 32. In lamps of this nature which maybe employed to produce either or both visible radiation and ultravioletradiation, the diffusing coating may be employed to diffuse the visibleradiation and to produce beneficial diffusion of the ultravioletradiation.

In Fig. 9, I have shown the invention as applied t0 the inner surface ofa glass globe 33 surround- 2. A glass electric lamp bulb having on theinside surface thereof a light-diffusing coating of particles ofamorphous silica of substantially spherical configuration and having anaverage diameter within the range from about A to 75 micron, saidcoating having a density ranging 14 from a. fraction of a milligram toseveral milligrams of silica per square inch.

3. A radiation diffusing body having a coating thereon consistingessentially of fine particles of amorphous silica having substantiallyspherical configuration and wherein the density of the coating is withinthe range from about 0.5 to about 20 milligrams per square inch.

4. A glass electric lamp bulb having on the inside surface thereof alight-diffusing coating of particles of amorphous silica ofsubstantially spherical configuration and having an average diameterwithin the range from about to micron.

5. A glass electric lamp bulb having on the inside surface thereof alight-diffusing coating of particles of silica self-adherent to theinside surface of said bulb and the numerical preponderance of whichhave a size less than the shortest wave length of light.

6. A glass electric lamp bulb having on the inside surface thereof alight-diffusing coating of particles of silica having an averagediameter of a fraction of a micron and a substantial number of whichparticles are fritted and self-adherent to said surface.

7. A glass electric lamp bulb having on the inside surface thereof alight-diffusing coating of self-adherent particles of silica having anaverage diameter within the range of about A, to micron.

8. A glass electric lamp bulb having on the inside surface thereof alight-diffusing coating of self -adherent particles of silica having anaverage diameter of the order of a half micron.

9. A glass electric lamp bulb having the inside surface thereof etchedand coated with a light-diffusing layer of particles of silica thenumerical preponderance of which have a size less than the shortest wavelength of light.

10. A glass electric lamp bulb having the inside surface thereof etchedand coated with a. light-diffusing layer of particles of silica havingan average diameter of a fraction of a micron.

11. A glass electric lamp bulb having the inside surface thereof etchedand coated with a light-diffusing layer of particles of silica having anaverage diameter within the range of about to /5 micron.

12. A glass electric lamp bulb having the inside surface thereof etchedand coated with a light-diffusing layer of particles of silica having anaverage diameter of the order of a half micron.

13. A glass electric lamp bulb having on its inner surface a coatingconsisting of extremely finely divided rounded particles of silica sothat the maximum brightness of an ordinary incandescent lamp comprisingsuch a coated bulb is of the order of a fraction of a per cent to a fewper cent of that of said lamp with a clear bulb.

14. A glass electric lamp bulb having its inner surface etched andprovided with a coating consisting of extremely finely divided roundedparticles of silica so that the maximum brightness of an ordinaryincandescent lamp comprising such a coated bulb is of the order of afraction (13f a per cent of that of said lamp with a clear ulb.

15. A light-transmitting electric lamp bulb having on the inside surfacethereof a light-diffusing coating consisting of a deposit of fumes of aninflammable silicon compound.

16. A glass electric lamp bulb having its inside surface etched andcoated with a light-difaccuses fusing layer consisting of a deposit offumes of an inflammable silicon compound.

17. An electric lamp having on the inside surface of a bulb therefor athin light-diffusing coating of finely divided particles of amo p silicaself-adherent to said surface, a filament mounted inside said bulb, anda gas filling consisting of about 98 per cent of an inert gas and 2 percent nitrogen by volume.

18. An electric lamp bulb having on the inside surface thereof a thinlight-diffusing coating of rounded particles of amorphous silica, asubstantial number of said particles being fused and self-adherent tosaid surface.

19. A glass electric lamp bulb having the inside surface thereof etchedand coated with a thin light-diffusing layer of amorphous silica.

20. The method of controlling the light-diffusing characteristics of alight-diffusing article having thereon a coating of finely dividedparticles of amorphous silica which comprises wetting at least a part ofthe silica coating to lessen the light-diffusing property of the partwhich is wetted.

21. In an incandescent lamp, the combination comprising a bulb, acoating of finely divided particles of silica on the interior surfacethereof, a filament mounted inside said bulb, and a gas fillingconsisting essentially of an inert gas of the group consisting of argon,krypton and xenon, or mixtures thereof, and an amount of nitrogen notexceeding 5 per cent by volume.

22. In an incandescent lamp, the combination comprising a glass bulb, acoating of finely divided particles of silica adherent to the interiorsurface of said bulb, and a filament mounted inside said bulb, saidcoating serving the dual function of a light-diffusing means and as ameans for preventing the dislodgment of any impurities from the insidesurface of the glass bulb which would tend to lower thebreakdown-voltage across said filament.

23. In an incandescent lamp, the combination comprising a glass bulb, alight diffusing coating of finely divided silica particles adherent tothe interior surface of said bulb, a filament mounted in said bulb, anda gas filling in said bulb comprising an inert gas, said coating servingas a means for holding any impurities occurring on the interior surfaceof the bulb.

24. In an incandescent lamp, the combination comprising a glass bulb, acoating of finely divided particles of silica on the interior surface ofsaid bulb, a filament mounted in said bulb, and a gas filling consistingessentially of an inert gas from the group of such gases having anatomic weight greater than 39, or mixtures thereof.

25. In an incandescent lamp, the combination comprising a bulb, acoating of finely divided rounded particles of amorphous silica on theinterior surface thereof, a filament mounted inside said bulb, and a gasfilling consisting essentially of argon.

26. In an incandescent lamp, the combination comprising a glass bulb, alight-diffusing coating of finely divided particles of silica on theinterior surface of said bulb, a filament mounted in said bulb, and agas filling in said bulb consisting essentially of an inert gas of thegroup consisting of argon, krypton and xenon, or mixtures thereof,

said coating serving as a means for preventing dislodgment of impuritiesfrom the interior surface of said bulb, and said inert gas permittingoperation of the filament at a higher temperature than that obtainableby use of gas fillings 16 other than the stated inert gases wherebyincreased luminous efllciency is obtained to recover any loss inefficiency due to said coating.

2'1. In an incandescent lamp, the combination comprising a glass bulb, acoating of finely divided rounded particles of amorphous silica adherentand fused to the interior surface of said bulb, a filament mountedinside said bulb, and a gas filling consisting of about 98 per centargon and 2 per cent nitrogen by volume.

28. An electric incandescent lamp comprising a sealed glass bulb havinga filament mounted therein, said bulb having its inside surface etchedand coated with a thin light-diffusing layer of finely divided roundedparticles of silica self-adherent to said surface.

29. An electric incandescent lamp comprising a sealed glass bulb havingits inside surface etched and coated with a thin light-diffusing layerof finely divided rounded particles of silica selfadherent to saidsurface, gas within said bulb of the group consisting of argon, krypton,xenon and mixtures thereof and a small amount of nitrogen not exceeding5 per cent by volume, and a filament mounted in said bulb operable attemperature greater than that in lamps of corresponding rating to affordrecovery of any loss in luminous efilciency occasioned by thelight-diffusing function of said layer.

30. In hollow illuminating glassware a radiation transmitting bodyhaving an etched surface and a coating of rounded particles of amorphoussilica directly and self-adherent to the etched surface.

31. An electric incandescent lamp comprising a sealed glass bulb havingits inside surface coated with a thin light-diffusing layer of finelydivided particles self-adherent to said surface and to preventdislodgement of any impurities from the inside surface of the bulb, gaswithin said bulb of the group consisting of argon, krypton, xenon andmixtures thereof and a small amount of nitrogen not exceeding 5 per centby volume, and a filament mounted in said bulb operable at temperaturegreater than that in lamps of corresponding rating to afford recovery ofany loss in luminous efiiciency occasioned by the light-diffusingfunction of said layer.

32. The method of treating a glass electric lamp bulb having an openingtherein to form a layer of light-diffusing material on the internalsurface thereof which comprises the steps of producing within theinterior of the bulb a combustible mixture of an organo-silicon compoundwhich is reducible by burning to silica, igniting the mixture andsupplying through said opening a steady stream of oxygen sufficient forcomplete combustion of the organo-silicon compound to therebydisassociate it into silica and other volatile products, continuing thecombustion and maintaining the fumes in contact with the inner surfaceof the bulb until a thin diffusing coating of silica particles isdeposited thereon while carrying the products of combustion other thansilica out through said opening.

MARVIN PlIKIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITE'DSTATES PATENTS Number Name Date 573,206 Chavez and Herman Dec.15, 1896 580,248 Biemann Apr. 6, 1897 (Other references on followingpage) 17 UNITED STATES PATENTS Number Name Date Hamburger and Lely Sept.30, 1919 Gustin Jan. 15, 1929 Fagan Nov. 3,1931 Fuwa. Apr. 19, 1932Ferguson July 5, 1932 Hageman et a1. Mar. 7, 1933 Biggs et a1. Apr. 23,1935 Bahlke et 'al. July 7, 1936 Parker Sept. 28, 1937 Weinhart Oct. 19,1937 Callan- Nov. 22, 1938 Heany Jan. 6, 1942 Hyde Feb. 10, 1942 NumberNumber Germany Mar. 20, 1934

